The present invention relates to a voltage boost system, and more particularly, to a voltage boost system that includes a feedback control circuit to stabilize the output of a voltage booster.
A voltage booster is used in electronic devices to generate different drive voltages from a sole power supply. A charge pump circuit, which includes a plurality of switching transistors and a capacitor, is one example of a voltage booster. The switching transistors are connected in series between an output terminal and a reference potential terminal of the charge pump circuit. One terminal of the capacitor is connected to a node between the series-connected transistors. FIG. 1 is a circuit diagram of a prior art charge pump circuit 50.
The charge pump circuit 50 includes switching transistors T1, T2, each of which are p-channel MOS transistors, a capacitor C1, and an output capacitor Cout. The capacitor C1 is provided with a cyclic clock CLK. Based on a pulse-height value VDD of the clock CLK, the capacitor C1 generates output voltage Vout, which has a maximum voltage of xe2x80x9cxe2x88x92VDDxe2x80x9d. FIG. 2 illustrates the transition of the output voltage Vout, node potential Vn1, and the clock CLK in the charge pump circuit 50.
To control the output voltage at a predetermined voltage, the charge pump circuit employs a circuit that feedback controls the clock signal based on the output voltage. The feedback control circuit normally includes a circuit for comparing the output voltage of the charge pump circuit with a certain reference voltage. Based on the comparison, the feedback control circuit controls the pulse number or pulse width of the clock signal applied to the charge pump circuit.
If the output voltage is lower than a desired value, the feedback control circuit increases the pulse number or the pulse width of the clock signal to increase the voltage boost rate of the charge pump circuit. The voltage boost rate is the increased amount of the charge pump circuit output voltage per unit time and takes a negative value when the output voltage decreases. If the output voltage exceeds the desired voltage, the feedback control circuit decreases the pulse number or the pulse width of the clock signal to decrease the voltage boost rate of the charge pump circuit.
The output voltage is controlled at the desired voltage by altering the voltage boost rate of the charge pump circuit based on the comparison between the output voltage of the charge pump circuit and a certain reference voltage.
Such feedback control generates the desired output voltage. However, a certain length of time is required for the output voltage to converge to the desired voltage. During the converging period, a fluctuating component of the output voltage produces an oscillation noise. The output voltage increases and decreases about the desired voltage.
Accordingly, when such voltage boost system is employed in, for example, a drive circuit of a CCD imaging device, the oscillation noise may be superimposed with an imaging signal. This may cause the noise to appear on a display.
The problem with noise applies not only to charge pump circuits but also to voltage boost systems that stabilize the output voltage of a voltage booster by performing feedback control.
It is an object of the present invention to provide a voltage boost system that smoothly converges the output voltage of a voltage booster when feedback controlling the output voltage.
To achieve the above object, the present invention provides a voltage boost system including a voltage booster for boosting an input voltage to generate a boosted output voltage. A feedback control Circuit is connected to the voltage booster to compare first and second voltages, which are based on one of an output voltage of the voltage booster and a reference voltage, with a third voltage, which is based on the other one of the output voltage and the reference voltage, to generate a feedback signal based on the comparison to feedback control the voltage booster. The feedback control circuit maintains the feedback signal at a constant value when the third voltage is included between the first and second voltages.
A further perspective of the present invention is a method for comparing an output voltage of a voltage booster with a reference voltage and feedback controlling a boost rate of the voltage booster in accordance with the comparison result. The method includes setting a first and a second voltages, which are based on one of the output voltage of the voltage booster and the reference voltage, comparing the first and second voltages with a third voltage, which is based on the other one of the output voltage of the voltage booster and the reference voltage, generating a feedback signal based on the comparison to feedback control the voltage booster, and maintaining the feedback signal at a constant value when the third voltage is included between the first and second voltages.
A further perspective of the present invention is a method for comparing an output voltage of a voltage booster with a reference voltage and feedback controlling a boost rate of the voltage booster in accordance with the comparison result. The voltage booster is connected to a drive circuit that drives a solid-state imaging device of an imaging apparatus. The solid-state imaging device generates an imaging signal including a horizontal scanning blanking period and a vertical scanning blanking period. The method includes setting a first and a second voltages, which are based on one of the output voltage of the voltage booster and the reference voltage, comparing the first and second voltages with a third voltage, which is based on the other one of the output voltage of the voltage booster and the reference voltage during at least either one of the horizontal scanning blanking period and the vertical scanning blanking period, generating a feedback signal based on the comparison to feedback control the voltage booster, feedback controlling the voltage booster based on the feedback signal during the blanking period, and maintaining the feedback signal at a constant value during the blanking period when the third voltage is included between the first and second voltages.